Are they all JPGs or NEF files?
If they are JPGs, you'd be better off writing a quick "desaturate"
Action and running the Image processor to batch them all.
if they are NEFs, you're laughing. Just use the LCH editor in Nikon
Capture on one image and then copy it to all the others before you
save them as JPGs

No. If you have NEFs and Nikon Capture, you just change the saturation back to normal directly (and probably the contrast too).